Catalytic oligomerization of olefins is a known technique for manufacturing hydrocarbon basestocks useful as lubricants. Efforts to improve upon the performance of natural mineral oil based lubricants by the synthesis of oligomeric hydrocarbon fluids have been the subject of important research and development in the petroleum industry for several decades, leading to recent commercial production of a number of superior poly(alpha-olefin) synthetic lubricants, hereafter called "PAO". These materials are primarily based on the oligomerization of alpha-olefins (l-alkenes), such as C.sub.6 -C.sub.20 olefins. Industrial research effort on synthetic lubricants has generally focused on fluids exhibiting useful viscosities over a wide range of temperature, i.e., improved viscosity index (VI), while also showing lubricity, thermal and oxidative stability and pour point equal to or better than mineral oil. These newer synthetic lubricants provide lower friction and hence increase mechanical efficiency across the full spectrum of mechanical loads and do so over a wider range of operating conditions than mineral oil lubricants.
Well known structural and physical property relationships for high polymers as contained in the various disciplines of polymer chemistry have pointed the way to l-alkenes as a fruitful field of investigation for the synthesis of oligomers with the structure thought to be needed to confer improved lubricant properties thereon. Due largely to studies on the polymerization of propene and vinyl monomers, the mechanism of the polymerization of l-alkene and the effect of that mechanism on polymer structure is reasonably well understood, providing a strong resource for targeting on potentially useful oligomerization methods and oligomer structures. Building on that resource, in the prior art oligomers of l-alkenes from C.sub.6 to C.sub.20 have been prepared with commercially useful synthetic lubricants from l-decene oligomerization yielding a distinctly superior lubricant product via either cationic or Ziegler catalyzed polymerization.
One characteristic of the molecular structure of l-alkene oligomers that has been found to correlate very well with improved lubricant properties in commercial synthetic lubricants is the ratio of methyl to methylene groups in the oligomer. The ratio is called the branch ratio and is calculated from infra red data as discussed in "Standard Hydrocarbons of High Molecular Weight", Analytical Chemistry, Vol. 25, no. 10, p.1466 (1953). Viscosity index has been found to increase with lower branch ratio. Prior, oligomeric liquid lubricants exhibiting very low branch ratios have not been synthesized from l-alkenes. For instance, oligomers prepared from l-decene by either cationic polymerization have branch ratios of greater than 0.20. Explanations for the apparently limiting value for branch ratio based on a cationic polymerization reaction mechanism involves rearrangement to produce branching. Other explanations suggest isomerization of the olefinic group in the one position to produce an internal olefin as the cause for branching. Whether by rearrangement, isomerization or other mechanism, l-alkene oligomerization to produce synthetic lubricants as produces excessive branching and constrains the lubricant properties, particularly with respect to viscosity index.
U.S. Pat. No. 4,282,392 to Cupples et al. discloses an alpha-olefin oligomer synthetic lubricant having an improved viscosity-volatility relationship and containing a high proportion of tetramer and pentamer via a hydrogenation process that effects skeletal rearrangement and isomeric composition. The product is a trimer to tetramer ratio no greater than 1:1.
A process using coordination catalysts to prepare high polymers from l-alkenes, especially chromium catalyst on a silica support, is described by Weiss et al. in Jour. Catalysis 88, 424-430 (1984) and in Offen. DE 3,427,319. The process and products therefrom are discussed in more detail hereinafter in comparison with the process and products of the present process.
It is well known that Lewis acids such as promoted BF.sub.3 and/or metal halides can catalyze Friedel-Crafts type reactions. However, olefin oligomers and more particularly PAO oligomers have been produced by methods in which double bond isomerization of the starting l-olefin occurs easily. As a result, the olefin oligomers have more short side branches. These side branches degrade their lubricating properties.
A significant problem in the manufacture of synthetic lubricants is the production of lubes in a preferred viscosity range in good yield without excessive catalyst deactivation. Frequently, it is difficult to directly produce lower viscosity range lube without incurring lower yields due to the production of non-lube range materials. Methods to control molecular weight of lubes in the oligomerization step are sought after in the art to overcome the problems in the manufacture of, particularly, lower viscosity lubes.